Biogenic substances of plants and animals, drugs, environment-related substances, and so forth have absorption bands in the near-infrared region. Thus, near-infrared spectroscopy has been receiving attention as a noninvasive analysis. Rapid advances have been made in the development and practical utilization of near-infrared spectroscopy. In particular, the issue of food safety has recently been receiving attention. As described above, absorption spectra of soybeans, rice, starch, and lipid, which belong to plants and animals, are observed in the near-infrared region. Thus, studies on the quality inspection of food using near-infrared spectroscopy have been actively conducted (NPL 1). In near-infrared spectroscopy, output signals contain necessary information and high levels of noise attributed to light receiving elements. The extraction of necessary information from output signals does not entirely depend on the improvement in the performance of sensors (light receiving elements) but is performed by spectroscopic methods, chemometrics, and so forth as important methods.
The foregoing sensors (light receiving elements) are broadly categorized into electron tubes and photodiodes (PDs), which are solid-state components. Among these components, PDs are small and easily integrated in the form of a linear array or a two-dimensional array. Thus, extensive research and development thereof have been conducted (NPL 2). The present invention aims at a food quality examination device including a PD. Nowadays, the following PDs and PD arrays are used.    (1) PDs having sensitivity up to the infrared region and also having sensitivity in the near-infrared region, or an array thereof. Examples of such photodiodes include germanium (Ge)-based PDs, lead sulfide (PbS)-based PDs, HgCdTe-based PDs, linear arrays thereof, and two-dimensional array thereof.    (2) InP-based PDs having sensitivity at wavelengths of 1.7 μm or less in the near-infrared region, InGaAs-based PDs included in InP-based PDs, and arrays thereof. The term “InP-based PDs” indicates PDs having absorption layers composed of III-V group compound semiconductors formed on InP substrates and includes InGaAs-based PDs.
Among these photodiodes, the PDs of item (1) are often cooled to suppress noise. For example, the PDs are often operated at the temperature of liquid nitrogen (77 K) or under cooling with Peltier elements. Thus, large-scale devices are needed, thereby increasing the cost of the devices. Although the PDs can be used at room temperature, the PDs disadvantageously have high dark currents at wavelengths of 2.5 μm or less and poor detection capabilities. The InP-based PDs of item (2) have disadvantages that (I) although InGaAs, which is lattice-matched to InP, has a low dark current, the sensitivity is limited to wavelengths of 1.7 μm or less in the near-infrared region, and (II) extended-InGaAs with sensitivity at extended wavelengths up to 2.6 μm has a high dark current and needs to be cooled. Thus, InP-based PDs cannot use light having a wavelength of 2.0 μm or more, the light being important in the inspection of food, or needs to be cooled when use the light.
light receiving elements that have been used for food quality inspection in the past are described below.    (C1) Methods of food quality inspection using lead sulfide (PbS) are disclosed (PTLs 1 to 4).    (C2) A measurement device using an InGaAs PIN photodiode is used (PTL 5).    (C3) Many literatures do not specifically describe what elements are used as infrared detectors (PTLs 6 to 13).
In the methods of food quality inspection described above, the sensitivities of light receiving elements themselves are not considered. All literatures suggest that what techniques should be used to perform the food quality inspection.
As described above, InGaAs PIN photodiodes have a problem of the need to extend the sensitivities to longer wavelengths in the near-infrared region. To solve the problem, the following measures are reported.    (K1) The In content of an InGaAs absorption layer is increased. A lattice-mismatch to an InP substrate is reduced by interposing a stepped buffer layer therebetween, the In content of the stepped buffer layer being gradually changed (PTL 14).    (K2) N is incorporated into an InGaAs absorption layer to form a GaInNAs absorption layer (PTL 15). A lattice matching to an InP substrate is satisfied by incorporating a large amount of N thereinto.    (K3) The use of a GaAs Sb-InGaAs type-II multiquantum well structure extends the sensitivity to longer wavelengths (NPL 3). The lattice matching to an InP substrate is satisfied.    (K4) A two-dimensional array is formed by forming element separation trenches between light receiving elements (pixels) using wet etching (PTL 16).
Although the measures for improvement are proposed as described above, the extension of the sensitivity to wavelengths of 1.7 μm or more and the suppression of noise and dark current have not been satisfactorily achieved by any of the measures of items (K1) to (K4).
NPL 1: Sumio Kawano, “Syokuhin no Hihakai Keisoku Handbook (Handbook of Nondestructive Measurement for Food)”, SCIENCE FORUM, pp. 34-40
NPL 2: Masao Nakayama, “Sekigaisen Kensyutsu Soshi no Gijutu Doukou (Technological Trends in Infrared Radiation Detection Elements)”, Sensor Gijutsu (Sensor Technology), March 1989 (Vol. 9, No. 3), pp. 61-64
NPL 3: R. Sidhu, “A Long-Wavelength Photodiode on InP Using Lattice-Matched GaInAs-GaAsSb Type-II Quantum Wells, IEEE Photonics Technology Letters, Vol. 17, No. 12 (2005), pp. 2715-2717
PTL 1: Japanese Unexamined Patent Application Publication No. 2005-233824
PTL 2: Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2003-510601
PTL 3: Japanese Unexamined Patent Application Publication No. 8-29336
PTL 4: Japanese Unexamined Patent Application Publication No. 8-29335
PTL 5: Japanese Unexamined Patent Application Publication No. 2007-93506
PTL 6: Japanese Unexamined Patent Application Publication No. 9-9767
PTL 7: Japanese Unexamined Patent Application Publication No. 5-232017
PTL 8: Japanese Unexamined Patent Application Publication No. 2001-4616
PTL 9: Japanese Unexamined Patent Application Publication No. 2007-212335
PTL 10: Japanese Unexamined Patent Application Publication No. 2007-225293
PTL 11: Japanese Unexamined Patent Application Publication No. 9-119894
PTL 12: Japanese Unexamined Patent Application Publication No. 9-250983
PTL 13: Japanese Unexamined Patent Application Publication No. 9-288056
PTL 14: Japanese Unexamined Patent Application Publication No. 2002-373999
PTL 15: Japanese Unexamined Patent Application Publication No. 9-219563
PTL 16: Japanese Unexamined Patent Application Publication No. 2001-144278